Method of reducing rutile



June 23, 1964 R. A. QUINN 3,138,504

METHOD OF REDUCING RUTILE Filed Sept. 15. 1960 FIG-2 INVENTOR. R088 A. QUINN BY Agent United States Patent 3,138,504 METHOD OF REDUCING RUTILE Ross A. Quinn, Palo Alto, Calif, assignor to Lockheed Aircraft Corporation, Burbank, Calif. Filed Sept. 15, 1960, Ser. No. 56,140 4 Claims. (Cl. 15618) a good insulator (10 ohm-centimeter) in the stoichiometric state to a conductor (0.1 ohm-centimeter) by varying the oxygen deficiency in the crystal lattice. Stoichiometric rutile with which this invention is concerned may be defined as a chemical composition of exact proportions by weight of titanium and oxygen in a tetragonal crystal lattice without any impurities, such as iron, for example.

Rutile is ordinarily reduced by heating a rutile sample in an oven having a reducing gas atmosphere in which hydrogen or some other reducing gas is circulated at various temperatures ranging from 450 to 1100 centigrade for various time intervals; for example, starting with a stoichiometric rutile crystal sample placed in a hydrogen atmosphere oven at a temperature of approximately 800 centigrade, 0.5 ohm-centimeter non-stoichiometric rutile will be obtained after reduction of the crystal for approximately one hour.

The reduction of rutile in an oven having a reducing gas atmosphere as described above is satisfactory for many applications, but as will be appreciated, such a method is not easily adaptable for semi-automatic or automatic techniques, chiefly because of the difliculties involved in providing an oven withan predetermined atmosphere in which the rutile must be inserted and removed. With the increased recognition of the advantages of using the properties of rutile for electronic applications, the desirability of providing an improved method of reducing rutile by semi-automatic or automatic techniques has taken on in creased importance.

Accordingly, it is an object of this invention to provide a method of reducing stoichiometric rutile which is much simpler and more convenient than presently known methods, and in addition is well suited for semi-automatic or automatic production techniques.

Another object of this invention is to provide a method of reducing stoichiometric rutile in which a high conductivity metal layer is also obtained on the surface of the ruti-ie by means of which an electrical connection can conveniently be made.

A further object of this invention is to provide a method of reducing stoichiometric rutile which may be carried out in a simple furnace under atmosphere conditions.

The present invention is based on my discovery that stoichiometric rutile may be reduced by subjecting it to a titanium coating process in a sandwich assembly by means of which a thin layer of a salt comprising any one or a mixture of the alkali or alkaline earth metals is sandwiched between a surface of the rutile and a layer of titanium or titanium alloy, and the resultant sandwich assembly then heated in air to a suflicient temperature and for a sufficient time to achieve the desired reduction of rutile.

The specific nature of the invention as well as other 3,138,504 Patented June 23, 1964 advantages, uses and objects thereof will clearly appear from the following description and the accompanying drawing in which:

FIG. 1 is a cross-sectional view of a three-layer sandwich assembly which is employed for reducing a stoichiometric rutile slab disposed therein in accordance with the invention.

FIG. 2 is a cross-sectional view of an extended modification of the assembly of FIG. 1 illustrating a five-layer sandwich assembly which may be employed for reducing a stoichiometric rutile slab disposed therein in accordance with the invention.

Like numerals designate like elements throughout the figures of the drawing.

Referring to FIG. 1, a three-layer sandwich assembly 30 rests on a support 10 which may suitably be made of graphite. One outer layer of the sandwich assembly 30 is a sheet 12 of titanium or titanium alloy resting on the support 16, the other outer layer is a slab 25 of stoichiometric rutile which is to be reduced, and the inner layer sandwiched therebetween is a salt 15 which may be any one or a mixture of the alkali or alkaline earth metal halides.

In order to provide the thin layer of salt 15 between the surface of the rutile slab 25 and the titanium sheet 12, a variety of techniques may be employed. The salt may be coated onto the titanium 12 and/ or the surface of the stoichiometric rutile slab 25, either by dipping into a molten salt bath, by spraying on a solution containing the salt, by melting on granules of the salt, or by spreading on a fine powder of the salt.

In accordance with the invention, the sandwich assembly 39 shown in FIG. 1 is now subjected to heat in an air environment which may be provided by any convenient means, such as a furnace, or just an open flame. The assembly 3G is heated to a sufiicient temperature to cause the salt 15 to melt and the titanium to deposit on the surface of the rutile slab 25. The thickness of the titanium film coated on the slab 25 will be dependent upon the temperature to which the assembly 30 is heated and the time for which the assembly is maintained at this temperature.

When the salt 15 melts during heating, the liquid boundary it forms between the titanium sheet 12 and the rutile slab 25 serves as a barrier to prevent oxidizing air surrounding the assembly 30 from being introduced to the surface of the rutile slab which it covers. As a result, the liquid barrier formed by the melted salt not only prevents oxidation from deleteriously interfering with the titanium deposition process, but also, I have discovered that the presence of the liquid salt barrier permits a reduction in the rutile slab to occur during the treatment, even though the assembly 30 itself is in an oxidizing air environment which would ordinarily preclude reducing action.

The reduction of the rutile slab 25 begins at the portion of the rutile slab 25 adjacent the deposited titanium film and expands outward therefrom as the time of treatment is increased. If a sufiicient time of treatment is provided, the rutile slab 25 may be reduced to a resistivity of less than .5 ohm-centimeter along with the deposition of a titanium film on a surface thereof. This titanium film may conveniently be employed to provide an electrical connection to the rutile slab. As is well known, there is a problem involved in soldering or otherwise making electrical contact to the rutile itself. On the other hand, if the titanium film on the rutile is not wanted, it can easily be removed by suitable etching techniques, such as by immersing the reduced rutile in a bath of dilute hydrochloric acid for a suificient time to remove the titanium film thereon.

The amount and thickness of the salt 15 required in a few molecular layers.

Because the surface tension of the molten salt will tend to maintain a liquid boundary between the titanium 12 and the rutile slab 25, even in the presence of excessive run-off or squeezing thereof, satisfactory results may be obtained with the sandwich assembly 30 in any position, even vertical. However, the dripping and run-off of salt is generally undesirable because it may get on apparatus or other units. It is preferred, therefore, that the salt layer be horizontal during heating, in which case the salt remains essentially between the sandwich and no clamping means need be provided. If the sandwich is vertical some means must be provided for holding the sandwich together during heating. Obviously, clamping should not be so great as to squeeze out so much of the melted salt out of the sandwich that a liquid boundary is not obtained during the titanium deposition process. Because of surface tension, however, this is usually no real problem in actual practice as long as the sandwich remains together.

Of course, until the salt melts, some air may be introduced into the system, but it has been found that a salt of one or a mixture of the alkali or alkaline earth metal halides melts at a sufficiently low temperature to prevent oxidizing air from interfering with the titanium deposition or the reduction of the rutile slab 25. Once the temperature is high enough so that oxidation could proceed rapidly, the salt has melted and the liquid boundary it forms bars the introduction of air.

In actual practice satisfactory titanium deposition and rutile reduction has been accomplished by uniformly spreading the salt 15 over a flat surface of the rutile which is to be reduced until the salt is observed to just cover the desired area on the rutile surface. The salt thickness is then ordinarily less than inch. Usually, salt thicknesses too much greater than M; inch are not desirable since excessive running of the salt may occur.

After the sandwich assembly 30 has been subjected to the temperature and for a time sufficient to produce the desired reduction of the rutile slab 25, the assembly 30 is cooled, the rutile slab separated and washed to remove any adherent salt. During cooling the molten salt 15 continues to bar the introduction of oxidizing air and becomes solid at a sufiiciently low temperature so that any air which might then be introduced when the salt is in solid form produces a negligible effect. As mentioned previously, the titanium film appearing on the reduced rutile may be used to permit convenient electrical connection to the rutile, or may be removed by etching.

If it is desired to speed up the rutile reduction action, or if it is desired that both surfaces of the rutile slab 25 have a titanium film thereon, a five-layer sandwich assembly 130 may be employed as shown in FIG. 2. As in the assembly of FIG. 1, the five-layer assembly 130 of FIG. 2 has a thin layer of salt 15 sandwiched between a surface of the rutile slab 25 and a layer of titanium 12. However, in the assembly 130 of FIG. 2, a second thin layer of salt 115 is sandwiched between the other surface of the rutile slab 25 and a second layer of titanium 112 as shown.

When'the five-layer sandwich assembly 130 of FIG. 2 is now subjected to heating as described in connection with FIG. 1, both surfaces of the rutile slab 25 will be coated with titanium and reduction of the rutile slab 25 will expand from both surfaces thereof. Thus, the time required to reduce a slab of rutile to a given extent will effectively be cut in half. Upon heating, both the salt 15 and the salt 115 melt to form liquid boundaries between .the surfaces of the rutile slab and the titanium sheets 12 A; and 112 adjacent thereto to keep out oxidizing air and permit titanium deposition and rutile reduction to occur without deleterious interference therefrom.

As in the three-element sandwich assembly 30 of FIG. 1, the weight of the various layers should not be such as to squeeze out so much salt that liquid boundaries might not be obtained when the salt melts. Since there is a greater possibility of running in the sandwich assembly 130 of FIG. 2, because of the larger amount of total salt, it may be desirable to provide short stubs 13 on the titanium layer 12 to prevent run-off.

A more complete understanding of the present invention may now be obtained from the following illustrative example. Referring to FIG. 1, a salt mixture 15 is selected essentially consisting of 22 mole percent NaCl, 53 mole percent KCl, and 25 mole percent KI. This salt mixture is then ground in a mortar and pestle to a fine intimately mixed powder which is dried for at least 24 hours before use at centigrade in a vacuum oven. The powder is then applied to the titanium sheet 12 by spreading it uniformly through a fine mesh screen until the powder appears to just cover the surface of the titanium 12. The resultant layer of the powder is then of the order of of an inch thick. The rutile slab to be reduced is now placed on the fine power and the sandwich assembly 30 is placed in a furnace at the temperature and for the time required to produce the desired reduction of the rutile slab. For example, a 4; inch thick stoichiometric rutile slab having an area of approximately one square centimeter can be reduced to a resistivity of the order of 0.1 5 ohm-centimeter by subjecting the sandwich assembly 30 of FIG. 1 in which this rutile slab is disposed to a temperature of 750 in an air atmosphere for a time of about 25 minutes. By suitably decreasing this time or lowering the temperature, it will be apprecited that any lesser reduction in the rutile can be obtained. Also, the particular temperature employed is not critical but preferably ranges from 450 to 1100 C. with the time of treatment chosen in accordance therewith.

It is to be understood that the particular embodiments and methods described herein are only exemplary and that many modifications in the steps and the construction and arrangement thereof are possible without departing from the spirit of the invention. The present invention, therefore, is to be considered as including all possible modifications and variations coming within the scope of the invention as defined in the appended claims.

I claim as my invention:

1. A method of partially reducing substantially stoichiometric rutile to change the electrical properties thereof which comprises sandwiching a thin layer of salt consisting essentially of at least one chosen from the group consisting of the alkali and alkaline earth metal halides between a layer of titanium and a surface of stoichiometric rutile which is to be reduced, and then heating the resulting sandwich assembly in an air environment to a temperature and for a time sufiicient to melt the salt and reduce the stoichiometric rutile by the amount desired, the thickness of the layer of salt being sufiicient to provide a liquid boundary between said layer of titanium and said surface of stoichiometric rutile.

2. A method of partially reducing a substantially stoichiometric rutile slab to change the electrical resistivity thereof which comprises sandwiching a thin layer of salt consisting essentially of at least one chosen from the group consisting of the alkali and alkaline earth metal halides between a flat surface of a layer of titanium and a surface of the stoichiometric rutile slab then heating the resulting sandwich assembly in an air environment to a temperature and for a time sufficient to melt the salt and reduce the stoichiometric rutile slab by the amount desired, the thickness of the layer of salt being sufficient to provide a liquid boundary between said layer and said surface of the rutile slab, allowing the sandwich assembly to cool, separating the stoichiometric rutile slab from the sandwich.

3. A method for changing the electrical insulating prop erties of a stoichiometric rutile slab by controllably decreasing the amount of oxygen deficiency in the crystal lattice thereof which comprises the steps of sandwiching a thin layer of salt consisting essentially of at least one chosen from the group consisting of the alkali and alkaline earth metal halides between the flat surface of a layer of titanium and the surface of the rutile slab, then heating the resulting sandwich assembly in an air environment to a temperature and for a time suflicient to melt the salt and decrease the amount of oxygen in .the rutile slab and by the desired amount, the thickness of the layer of salt being sufiicient to provide a liquid boundary between said layer and said surface of the rutile slab, allowing the sandwich assembly to cool, separating the rutile slab from the sandwich, and then etching away the titanium film formed on the surface of the rutile slab during said heating.

4. A method for controllably decreasing the oxygen content in a crystal lattice of a stoichiometric rutile slab to thereby change said slab from an electrical insulator to an electrical conductor, which comprises forming a fivelayer sandwich assembly made up of a first layer of titanium, a first thin layer of salt, a rutile slab, a second thin layer of salt, and a second thin layer of titanium, in

that order, the salt making up said layer consisting essentially of at least one chosen from the group consisting of the alkali and alkaline earth metal halides, then heating the sandwich assembly in an air environment to a temperature and for a time suflicient to melt the salt and re duce the rutile by an amount desired, the thickness of the first and second salt layer being sufficient to provide liquid boundaries between the opposite surfaces of the rutile slab and the titanium layers adjacent thereto, allowing the sandwich assembly to cool, separating the reduced rutile slab therefrom and then etching away the titanium film formed on .the surface of the rutile slab during said heatmg.

References Cited in the file of this patent UNITED STATES PATENTS 2,289,211 Ridgway July 7, 1942 2,369,266 Thurnauer Feb. 13, 1945 2,807,539 Quin Sept. 24, 1957 2,834,667 Rostron May 13, 1958 2,886,682 Martin May 12, 1959 3,022,201 Quinn et al. Feb. 20, 1962 3,044,968 Ichikawa July 17, 1962 3,105,800 Watanabe Oct. 1, 1963 

1. A METHOD OF PARTIALLY REDUCING SUBSTANTIALLY STOICHIOMETRIC RUTILE TO CHANGE THE ELECTRICAL PROPERTIES THEREOF WHICH COMPRISES SANDWICHING A THIN LAYER OF SALT CONSISTING ESSENTIALLY OF AT LEAST ONE CHOSEN FROM THE GROUP CONSISTING OF THE ALKALI AND ALKALINE EARTH METAL HALIDES BETWEEN A LAYER OF TITANIUM AND A SURFACE OF STOICHIOMETRIC RUTILE WHICH IS TO BE REDUCED, AND THEN HEATING THE RESULTING SANDWICH ASSEMBLY IN AN AIR ENVIROMENT TO A TEMPERATURE AND FOR A TIME SUFFICIENT TO MELT THE SALT AND REDUCE THE STOICHIOMETRIC RUTILE BY THE AMOUNT DESIRED, THE THICKNESS OF THE LAYER OF SALT BEING SUFFICIENT TO PROVIDE A LIQUID BOUNDARY BETWEEN SAID LAYER OF TITANIUM AND SAID SURFACE OF STOICHIOMETRIC RUTILE. 